Shares in Amazon jumped 15% in after-hours trading in New York after the online retailer reported a big increase in sales on the back of continued popularity of the Kindle Fire.

Amazon's net sales leaped 34% to $13.18 billion (£8.16bn) in the three months to March 31, up from $9.86 billion in the first quarter of 2011.

The Kindle Fire, Amazon's tablet computer that is still not available in the UK, remains the best-selling product across millions of items on Amazon.com, a position it has held since launching in the US last autumn.

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The seven-inch screen toting colour tablet is considered to be a key competitor to Apple's iPad due to its cheap price, along with Amazon's ability to offer a big library of digital content.

In the first quarter, nine out of the ten top-selling items on Amazon.com were digital products, including Kindle devices (tablet and e-reader), Kindle eBooks, movies, music and apps.

Amazon's North America and Canada sales were $7.43 billion in the first quarter, up 36% year-on-year, while international sales were $5.76 billion, up 31% on the same period in 2011. Amazon said that worldwide sales from media services grew 19% to $4.71 billion.

Net income during the quarter actually fell 35% to $130m (£80m), from $201m, but that was primarily down to further increases in investment.

Amazon has been spending heavily on its distribution centres to support online sales, as well as video content deals and infrastructure to support businesses such as cloud computing.

However, investors have been impressed after its overall revenues and earnings beat analyst forecasts, suggesting that the investments are starting to pay off.

The leap in share price boosted Amazon's market value by more than $10bn (£6.18bn).

During the quarter, Amazon repurchased 5.3 million shares, or $960 million, under its previously announced intention to repurchase up to $2bn of the company's common stock.

Amazon said that it expects second quarter earnings of between $11.9 billion and $13.3 billion, growth of between 20% and 34% compared with second quarter 2011.